The present invention relates to a process for converting hydrogen and carbon monoxide into hydrocarbon products. More particularly, the present invention relates to processes for conversion of hydrogen and carbon monoxide into C.sub.1 -C.sub.4 range saturated hydrocarbons, particularly methane, in the presence of an iron based catalyst having improved activity and selectivity for production of C.sub.1 -C.sub.4 range hydrocarbons from hydrogen and carbon monoxide.
Processes for production of gas streams comprising hydrogen and carbon monoxide from carbonaceous and/or hydrocarbon charge stocks are well known in the prior art. Of particular interest for providing hydrogen and carbon monoxide charge for the process of the present invention, is partial oxidation of hydrocarbons and/or carbonaceous charge stocks with a molecular oxygen-containing gas. Such partial oxidation processes are widely practiced on a commercial basis. In partial oxidation reactions, hydrocarbons or carbonaceous charge stocks, ranging from methane through coal or coke, are contacted with an amount of molecular oxygen insufficient for complete combustion of such charge stock, at temperatures in the range of about 1600.degree. F. to about 3000.degree. F. and pressures in the range of about 1-250 atmospheres, for conversion of the oxygen and charge stock into hydrogen and carbon monoxide. It is known to charge water or steam to partial oxidation reactions for increasing the ratio of hydrogen to carbon monoxide in the product gas. Also, it is known to maintain the reaction temperature within the range of about 1600.degree.-2000.degree. F. and to add water or steam to the partial oxidation reaction for production of a gas product comprising substantial amounts of methane as well as hydrogen and carbon monoxide. As no catalyst is employed in the partial oxidation reaction, it is not necessary to remove sulfur and other materials from the charge stock prior to the partial oxidation reaction. Sulfur is primarily converted to hydrogen sulfide (H.sub.2 S) and minor amounts of carbonyl sulfide (COS). In addition to hydrogen, carbon monoxide and methane, a minor portion of the hydrocarbon or carbonaceous charge stock is converted to carbon dioxide and about 0.5 to 5 weight percent is converted to soot. Processes for removal of impurities from partial oxidation effluent gases and production of gases substantially comprising hydrogen and carbon monoxide are well known in the prior art and are widely practiced.
The molar ratio of hydrogen to carbon monoxide in a product gas from a partial oxidation process may be increased substantially by subjecting such gas to the water gas shift reaction. The water gas shift reaction comprises heating the gas to a temperature of about 550.degree. F. or higher and contacting said gas with steam in the presence of a suitable catalyst. Under such conditions, carbon monoxide reacts with steam to form carbon dioxide and hydrogen. Carbon dioxide formed from the water gas shift reaction may be removed from the product gas by acid gas absorption techniques.
It is becoming increasingly desirable to convert low value hydrocarbon and carbonaceous fuels into more useful fuel products, particularly methane and low molecular weight C.sub.1 -C.sub.4 range saturated hydrocarbons. Particularly, it is desirable to convert hydrocarbonaceous or carbonaceous charge stocks containing high percentages of impurities such as ash and sulfur into clean products which may be burned as fuels without contributing substantial pollution of the environment. Partial oxidation processes produce product gasses comprising hydrogen and carbon monoxide which may be efficiently treated for removal of impurities which may lead to air pollution. However, the resulting gas products, substantially comprising hydrogen and carbon monoxide, have a relatively low heating value particularly when compared to naturally occurring fuel gases such as methane. Therefore, conversion processes for reacting hydrogen with carbon monoxide for conversion into low molecular weight saturated hydrocarbons, particularly methane, have been developed. In such processes hydrogen and carbon monoxide in a molar ratio of from about 1:1 to about 4:1 and preferably in the range of about 1:1 to about 3:1 are contacted at pressures in the range of about atmospheric to about 25 atmospheres and higher, and temperatures in the range of about 600.degree. F. to about 900.degree. F., in the presence of selected catalysts for conversion of hydrogen and carbon monoxide into low molecular weight saturated hydrocarbons. Catalysts which may be employed in such reactions include metals, preferably in subdivided form, such as iron, cobalt, nickel, vanadium, molybdenum, and tungsten, and compounds of these metals such as the halides, oxides, sulfides, molybdates, sulfates, or oxylates. Mixtures and other combinations of two or more of these metals and/or compounds of these metals may be employed as desired. Exemplary of such prior art processes for conversion of hydrogen and carbon monoxide is the process disclosed in U.S. Pat. No. 3,730,694, D. K. Wunderlich, issued May, 1973. In such processes, the heat of reaction from the conversion of hydrogen and carbon monoxide is substantial and means for removing such heat of reaction must be provided to prevent a rapid increase in temperature which, if uncontrolled, would damage the catalyst and/or damage the processing equipment. Methods for controlling the temperature in such conversion reactions, include recycling inert gasses, such as a portion of the product gas, and providing indirect heat exchange such as steam coils, within the catalyst bed. One known method for removing the heat of reaction and thereby controlling the conversion reaction temperature is to employ catalyst in finely subdivided form (less than 100 microns) and fluidizing said catalyst with gas charged to the process to form a dense phase fluidized bed. Heat exchange means, such as steam generation coils, are suspended within said fluidized bed for absorption of the heat of reaction. Such fluidized beds have very good heat transfer properties which allow rapid, even transfer of heat from the catalyst bed to the heat removal means, e.g., the steam coils.
The more reactive catalysts for conversion of hydrogen and carbon monoxide, particularly those containing nickel, are poisoned by sulfur and sulfur compounds. Such poisoning results in the catalysts losing activity for conversion of hydrogen and carbon monoxide. Whereas, catalysts which are more tolerant of sulfur, particularly iron containing catalysts, have lower initial activity for conversion of hydrogen and carbon monoxide. Therefore, in processes of the prior art, hydrogen and carbon monoxide charged to such conversion reactions are commonly treated for substantially complete removal of sulfur and sulfur compounds if a highly active nickel-based catalyst is to be employed. Otherwise, use of more sulfur tolerant catalysts results in lower conversion of hydrogen and carbon monoxide into desirable low molecular weight hydrocarbon compounds.